The present invention relates to a process for preparing polyolefins using a specific catalyst. Particularly, the present invention is concerned with a process for preparing polyolefins, capable of preparing the polyolefins in a high polymerization activity by homo- or copolymerization of olefins in the presence of a specific catalyst, and capable of freely controlling the molecular weight distribution of the polyolefins obtained, the polyolefins having superior characteristics, for example a narrow composition distribution particularly in the case of copolymerization of ethylene and .alpha.-olefin.
Using zirconium compounds (typically metallocene compounds) in the preparation of polyolefins, especially homopolymers of ethylene or copolymers of ethylene and .alpha.-olefin, is known as disclosed in Japanese Patent Laid Open No. 19309/1983. However, although the process using such solid catalyst components can afford ethylene copolymers in high yield, the copolymers obtained are narrow in molecular weight distribution, also narrow in composition distribution, and low in molecular weight.
The molecular weight of resulting polymer can be increased to some extent by selecting a suitable transition metal compound of metallocene which is one component of the catalyst used. For example, in Japanese Patent Laid Open No. 234005/1988 it is proposed to use a transition metal compound having 2, 3 and 4-substituted cyclopentadienyl for increasing the molecular weight of the resulting polymer. Further, in Japanese Patent Laid Open No. 22307/1990 it is proposed to use a hafnium compound having a ligand bonded to at least two crosslinked conjugated cycloalkadienyl groups for increasing the molecular weight of the resulting polymer.
However, the preparation of such proposed catalyst components is complicated in both synthesis route and operation, and the use of hafnium as a transition metal is disadvantageous in that the yield of the resulting polymer is low.
It is the object of the present invention to overcome the abovementioned drawbacks, particularly provide a polyolefin preparing process which permits simplifying the preparation of catalyst components, permits easy control of the molecular weight distribution and can afford a easily polyolefin having a narrow composition distribution and high molecular weight in high yield.